1. Field of the Invention
The present invention relates to an optical displacement-detecting mechanism used in a scanning probe microscope, a surface topography measuring apparatus with a probe, and the like, which irradiates a target for measurement, e.g. a cantilever for a scanning probe microscope, with light from a light source, and detects the intensity of light after irradiation by use of an optical detector made from a semiconductor thereby to detect the displacement of the measurement target.
2. Description of the Related Art
A scanning probe microscope (SPM: Scanning Probe Microscope) has been known as an apparatus for measurement of a micro-scale area of a sample, e.g. metal, semiconductor, ceramic, resin, polymeric material, biomaterial, and insulating material, and for observation of an asperity image of a sample surface and information about a physical property thereof.
As a SPM, a microscope including a sample holder to put a sample on, and a cantilever with a probe attached on a tip thereof has been known well, in which the probe is brought close to the sample in use. With such scanning probe microscope, a surface topography and various kinds of physical property information are measured by: relatively moving a sample and the probe in a sample plane (X-Y plane) to scan a surface of the sample; and moving the sample or probe in a direction (Z direction) orthogonal to the sample surface while measuring the quantity of displacement of the cantilever with a displacement-detecting mechanism during scan, thereby to control the distance between the sample and probe.
A configuration of a typical, conventional scanning probe microscope is shown in FIG. 7 (see e.g., JP-A-10-104245).
In the scanning probe microscope 201 shown in FIG. 7, a sample 211 is moved finely in a direction (Z direction) perpendicular to a sample plane (X-Y plane) while the sample 211 is scanned in the sample plane by means of a three-axis micro-moving mechanism (scanner) 213. The three-axis micro-moving mechanism is composed of a cylindrical piezoelectric device having a top end with a sample stage 212 to put a sample on and a bottom end fixed on a base 215.
In addition, a cantilever 207 with a probe 209 on the tip thereof is held on an arm 205 of a high rigidity; the arm is attached to a support rod 203 fixed to the base 215. On a lower face of a tip portion of the cantilever 207, the probe 209 is formed protruding downward. Hence, the tip of the probe 209 can be brought close to a surface of the sample 211 by a roughly moving mechanism (not shown) which is operable to move the probe in Z direction.
An optical displacement-detecting mechanism is provided above the cantilever 207, which includes a semiconductor laser (LD) 221 and an optical detector 235 made from a semiconductor and which is termed an optical lever system in general.
Now, the operational principle of an optical displacement-detecting mechanism of the optical lever system will be described in detail. (See e.g. Takeshi Fukuma et al., “Development of Low Noise Cantilever Deflection Sensor for Multi Environment Frequency-modulation Atomic Force Microscopy”, REVIEW OF SCIENTIFIC INSTRUMENTS, 76, 053704 (2005)).
FIG. 6A is an illustration showing a configuration of an optical displacement-detecting mechanism 200. FIG. 6B is a diagram of an electric circuit connected with an optical detector 235 made from a semiconductor. The optical displacement-detecting mechanism 200 launches a laser beam (incident light 231) from the light source 221, which is placed above the cantilever 207 and composed of a semiconductor laser, while focusing the laser beam on a rear face of the cantilever 207 through a lens 240. The incident light 231 is reflected off the rear face of the cantilever 207. The reflected light 233 impinges on the optical detector 235, which is placed above the cantilever 207 in an oblique direction with respect to the rear face of the cantilever and made from a semiconductor. The optical detector 235 has a light-receiving face divided into two, upper and lower halves (areas A and B), and is arranged so that an incident position where the reflected light 233 impinges on the detector can be detected.
When light impinges on the light-receiving face (the areas A and B) of the optical detector 235, electric currents iA and iB are respectively generated there. Behind the light-receiving face, current-voltage conversion circuits 242a and 242b are connected with the light-receiving areas respectively. The current signals iA and iB are converted into voltage signals vA and vB with amplification factors depending on the feedback resistance values RIV. The voltage signals are input to a differential amplifier circuit 243, which is to be described later.
In the case of the optical displacement-detecting mechanism shown in FIGS. 6A, 6B and 7, when the probe 209 and sample 211 are brought close to each other, an atomic force acts initially. When the probe and sample are brought closer to each other, a contact force acts, causing deflection in the cantilever 207. The deflection of the cantilever 207 shifts a spot 241 on the light-receiving face of the optical detector 235 upward or downward. The differential amplifier circuit 243 detects the difference vA-B of voltage signals from the upper and lower light-receiving face areas A and B, whereby the quantity of deflection of the cantilever 207 can be measured. Usually a band-pass filter 244 is provided downstream of the differential amplifier circuit 243 for the purpose of cutting frequency components outside the band used for measurement thereby to hold down noises. A signal which has gone through the band-pass filter 244 is sent to a Z feedback circuit 251.
The quantity of deflection of the cantilever 207 depends on the distance between the probe 209 and a surface of the sample 211. Therefore, an asperity image of a sample surface can be obtained by: detecting the quantity of deflection of the cantilever 207 in the form of an output voltage vA-B of the optical detector 235; inputting the quantity of deflection to the Z feedback circuit 251; controlling the distance between the probe 209 and the surface of the sample 211 by means of the Z micro-moving mechanism 213 so that the quantity of deflection is made constant, i.e. the output voltage VA-B is made constant; and using an XY scanner 213 to scan the sample. The control is performed by the control section 257. The three-axis micro-moving mechanism 213 is driven by the XYZ scanner driver 253. An asperity image thus obtained is displayed by a display section 255.
As for the optical displacement-detecting mechanism, the resolution of measured data in a direction of the height of a sample is determined by the detection sensitivity of the displacement-detecting mechanism (i.e. the quantity of an output voltage per unit length) and the intensities of noise components mixing in a signal from the optical displacement-detecting mechanism.
There are some contributing factors to noises in the optical displacement-detecting mechanism (see supra “Development of Low Noise Cantilever Deflection Sensor for Multi Environment Frequency-modulation Atomic Force Microscopy”). The factors are as follows.
1. Shot noise coming from the optical detector
2. Johnson noise (thermal noise) coming from the optical detector
3. Quantum-mechanical noise coming from the light source
4. Optical feedback noise and Mode hop noise caused by the light source
5. Thermal fluctuation of the cantilever
6. Interference noise of light
Among the factors, what contributes to noises in the optical displacement-detecting mechanism at the highest degree of dependence in a frequency band used by a typical scanning probe microscope is the shot noise attributed to the optical detector of the first noise factor described above. The percentage of the shot noise affecting the detection sensitivity becomes smaller in inverse proportion to the square root of a light quantity P in the light-receiving face.
As a frequency at which the measurement is performed is shifted to a higher region, the degree of dependence on Johnson noise of the second noise factor described above increases. The percentage of the Johnson noise affecting the detection sensitivity becomes smaller in inverse proportion to the light quantity P in the light-receiving face.
The light quantity P in the light-receiving face is given by P=αP0, where P0 represents an output of the light source, a represents a light transmission efficiency of an optical path from the light source to the optical detector through a target for measurement.
As described above, with the shot noise and Johnson noise, when the intensity of light, i.e. light quantity P, in the light-receiving face of the optical detector increases, the quantity of noise with respect to the detection sensitivity decreases, and thus the resolution of measured data is enhanced. In other words, for the purpose of decreasing the percentage of noise with respect to the detection sensitivity, it is useful to increase the output P0 of the light source or to increase the transmission efficiency of the optical path.
Now, light source noises of a semiconductor laser, which is a light source used in a conventional optical displacement-detecting mechanism most commonly, will be examined. In a semiconductor laser, the percentage of the spontaneously emitted light increases inside the device in a low-power region, and thus the noise which is termed quantum-mechanical noise of the third noise factor described above is generated. The percentage of induced emission light becomes dominant with an increase in the laser power, whereby the percentage of quantum-mechanical noise is reduced. As for a semiconductor laser, the larger the output is, the smaller the quantum-mechanical noise is, whereas in the case of driving the laser with a high power, the optical feedback noise which is caused by light reflected by a cantilever, a sample, an optical device placed on the optical path and the like and fed back to the semiconductor laser, and mode hop noise which is generated when the temperature or power of the laser varies are developed as described above in the fourth noise factor. Hence, the light source has an optimal value in its output. Therefore, a semiconductor laser has been driven with a power of 2 mW or smaller in the art. As described above, it is required for reduction in the level of quantum-mechanical noise of an optical detector to increase the output of the light source. However, the output of the light source is restricted by suppressing the generation of optical feedback noise and mode hop noise on the side of the light source.
In addition, to reduce the mode hop noise and optical feedback noise, it is effective to lower the coherency of the light source. In other words, it is preferable to use a light source having a wide spectrum width in a portion where the maximum intensity arises in the intensity spectrum vs. wavelength of the light source. For this purpose, a semiconductor laser has been modulated with a high frequency.
Further, for the purpose of reducing the mode hop noise and optical feedback noise, the measure of using an optical system that the polarization states of incident light and reflected light are changed thereby to avoid the feedback of the reflected light to the semiconductor laser has been taken to prevent optical feedback caused by a target for measurement, a member on the optical path and the like.
Semiconductor lasers have a high coherency and are a light source superior in coherence. Therefore, with regard to e.g. a scanning probe microscope, in some cases interference of the light reflected by a cantilever with the light which has bulged out of the range of the cantilever and undergone reflection by a sample causes the interference noise in an asperity image and data obtained when a physical property with respect to the distance between the probe and a sample is measured as described above in the sixth noise factor.
However, even when high frequency modulation and light polarization by an optical system are utilized, the mode hop noise and optical feedback noise cannot be held down perfectly. Hence, in the art the output of a semiconductor laser has been made 2 mW or smaller, whereby a light source has been driven in a region where the mode hop noise and optical feedback noise are hard to cause.
To make use of the high frequency modulation and polarization optical system, it is required to prepare a special circuit and a special optical device, which makes the apparatus more complicated and increases the cost.